Telecommunications service providers continue to seek ever greater bandwidth at ever lower prices. Their data networks must be flexible to allow for continual upgrades, also referred to as “provisioning”. They must also designed for rapid fault recovery to avoid service degradation and even outages. High speed optical data networks now carry most of the long haul, and much of the metropolitan area data traffic in developed countries. Along such networks microprocessor controlled routers perform so-called “OEO” transcriptions, converting optically encoded data received from input optical fibers to electrical signals, reading destination code, and then reconverting the electrical signals back to optically encoded data and sending it along output optical fibers. As transmission speeds pass 2.488 Gbits/sec (OC-48 level), this conversion step becomes more difficult to perform and the cost of conventional high throughput electrical switches becomes unacceptable.
Pure optical switches direct light pulses directly from one optical fiber to another without electrical conversion and therefore offer the promise of eliminating much of the OEO transcriptions in high bandwidth fiber optic data transmission networks. Electrical routing intelligence would still be needed to direct traffic. However, currently about eighty percent of the traffic handled by a conventional router passes straight through and reading the destination header in most cases is a waste of time and system resources, By separating the control information from the transmitted data, pure optical switching would bring substantial increases in the throughput rate of optical data networks.
In general a pure optical switch routes beams of light with encoded data from one or more input optical fibers to a choice of two or more output optical fibers. Fiberoptic switches are often described by the number of channels that they are capable of switching. For example, a 1×4 fiberoptic switch has one input fiber whose information light beam can switched among four different output optical fibers. A 4×4 fiberoptic switch allows switching between four input fibers and four output fibers. In a so-called “non-blocking fiberoptic matrix switch” any of the inputs can be connected to any of the outputs, provided that each input is only connected to one output.
A variety of miniature electromechanical devices have been developed for changing the path of light in free space to direct light pulses from one optical fiber to another optical fiber. One promising approach utilizes three dimensional (3D) microelectromechanical systems (MEMS). Generally speaking, MEMS fabrication technology involves shaping a multi-layer monolithic structure by sequentially depositing and configuring layers of a multi-layer wafer. The wafer typically includes a plurality of polysilicon layers that are separated by layers of silicon dioxide and silicon nitride. The shaping of individual layers is done by etching that is controlled by masks patterned by photolithographic techniques. MEMS fabrication technology also entails etching intermediate sacrificial layers of the wafer to release overlying layers for use as thin elements that can be easily deformed and moved. Further details of MEMS fabrication technology may be found in a paper entitled “MEMS The Word for Optical Beam Manipulation” published in Circuits and Devices, July 1997, pp. 11–18. See also “Multiuser MEMS Processes (MUMPS) Introduction and Design Rules” Rev. 4, Jul. 15, 1996 MCNC Mems Technology Applications Center, Research Triangle Park, N.C. 27709 by D. Keoster, R. Majedevan, A. Shishkoff and K. Marcus.
Optical switches that employ moving mirrors have several drawbacks. They can have large insertion losses resulting from beam divergence between the input and output fibers. This loss scales upwards as the number of channels increases since larger propagation distances are required. Precise angular alignment of the mirrors is also required to minimize optical losses. In addition, the long term reliability of micro-machined tilting mirrors has yet to be firmly established. Furthermore, in some designs, bi-stable mirrors can maintain their state in the absence of power, however, many optical switch designs that employ moving mirrors require continuous power to maintain a fixed state.
Micro-fluidic total internal reflection optical switches have also been developed. Two arrays of optical waveguides cross each other. A fluid filled trench is created at each crossing point. If fluid is present at an interface, light from and incident waveguide will propagate across the trench, continuing along the same path as that of the incident waveguide. If a bubble is present at the interface, then light from the incident waveguide will be reflected by total internal reflection, thus coupling into another waveguide belonging to the array crossing the incident array. This optical switch design has several disadvantages. Optical losses accrue each time light is transmitted across a fluid filled trench. The total optical losses scale upwards in proportion to an increasing number of channels. Furthermore, the micro-fluidic optical switch requires a continuous source of power to maintain a fixed state. The long term reliability and environmental stability of micro-fluidic optical switches has yet to be firmly established.
Acousto-optic waveguide optical switches have also been developed. However, they require an inordinately large number of 1×2 cascading units, making them costly and generating substantial insertion losses. They also require power to maintain a fixed state.
Lens-based mechanical fiberoptic switches have also been developed. Light from an input fiber is collimated by a lens and then focused by a second lens onto an output fiber. In one version, an array of output fibers can be translated mechanically to select one fiber in the array to be at the focal point of the focusing lens. In another version, a mirror is rotated to reflect the collimated input beam to a selected output lens, thus focusing the beam into a selected output fiber. This type of switch is limited to 1×N configurations and the precision optical alignment of all of the fibers and lenses is required, greatly increasing the cost of this type of optical switch.
Proximity-based mechanical fiberoptic switches have also been designed in which an input fiber is in close proximity to a moveable array of output fibers. By translating the output array, an output optical fiber can be selected. This design suffers from the drawback that it is limited to 1×N configurations.
Of course, mechanical fiberoptic patch panels have existed which require a human operator to physically plug and unplug ferruled optical fibers into appropriate sockets. The severe disadvantages in speed and efficiency of this type of crude fiberoptic switch are readily apparent.
It would therefore be desirable to provide a reliable N×N non-blocking mechanical fiberoptic matrix switch. Its switching speed would be significantly less than that of some of the other pure optical switches identified above. However, there is still a need for this type of fiberoptic switch where switching speed is not critical and channel re-routing occurs very infrequently. This can occur, for example, where new equipment is being brought on line, data paths are being re-routed to bypass defective equipment or add capacity, and so forth. Furthermore, there is a need for an N×N non-blocking fiberoptic matrix with very low insertion losses.